The invention disclosed herein relates generally to an apparatus and method for obtaining a slurry with a desired density during a continuous-mixing operation. More particularly, the invention relates to an automated system for automatically controlling the density of a cement slurry with a very high degree of precision for use in well completions.
Systems for mixing dry cement with water are well known in the art. Typically, cement mixing for large jobs is either done by batch-mixing or by continuous-mixing. A batch-mixing operation occurs when the ingredients are mixed in a large tank or blender to obtain the entire volume of cement slurry before the start of the job. A continuous-mixing job, conversely, is an operation in which the ingredients are continuously mixed during the course of the job to produce a slurry for immediate use. The advantages of batch-mixing cement is that the density can be controlled very accurately. The disadvantage is that batch-mixing may prove to be impossible or impractical on large jobs in which a large volume of cement slurry must be generated. Because the slurry is premixed in a batch job, a blender or tank must be provided that is large enough to hold all of the slurry to be used on that particular job. Continuous mixing alleviates this problem, in that the slurry is mixed "on the fly" in a relatively small mixing chamber and is used immediately.
One problem, however, with continuously mixing the slurry is that it is very difficult to control the density of the slurry with any degree of precision because ingredients are constantly being added and slurry is constantly being discharged. As a result, it is common to have fluctuations in slurry density during continuous-mix operations. In certain applications, cement density fluctuation can cause severe problems. One example where cement density fluctuations are particularly undesirable is in cementing operations for casing a wellbore. The density of cement is especially critical for such cementing operations.
Cement is used in wells to secure casing in place in a wellbore to "complete" the well. The purpose of the cement is to seal and block various zones between the casing and the wellbore. Special additives may be mixed with the cement to alter specific properties of the cement, as required by the wellbore and casing characteristics and relationships. A general overview of cementing casing may be found in Skinner, D. R., Introduction to Petroleum Production, Volume I, Chapter 4: Well Completion (Gulf Publishing Co. 1981), and in Moore, Preston L., Drilling Practices Manual, Chapter 16 (PennWell Publishing Co. 1974).
Several terms commonly are used in cementing operations, as follows:
Cement Slurry refers to the mixture of dry or powdered cement and water that is injected or pumped into the wellbore;
Slurry volume refers to the volume of slurry that is obtained when a given volume of dry cement is mixed with a given volume of water; and
Slurry density is the weight of a given volume of mixed slurry, and typically is measured in terms of pounds per gallon (also referred to as "PPG");
Different cements cure in different ways; for example, some cements expand as they cure, while others shrink. During the curing process, cement generally increases in temperature. Some cement mixtures will become weak or increasingly permeable as a result of this increase in temperature during the curing process. Because the heat of the earth increases at greater depths, degradation of the cement becomes more pronounced as the depth of the wellbore increases if cement is used downhole in wells.
The cement and water typically are mixed on site during a cementing operation because most wells are located in remote locations where it is impractical to use large mixing tanks. Such an application commonly is referred to as a continuous-mixing job. The materials used in the cement are usually prepared dry and transported to the well site, where it is mixed with liquid or "mix water" and pumped into the well. Various dry or liquid additives also may be added to either the mix water or to the dry cement as desired to alter the properties of the cement slurry. The cement slurry normally is pumped in liquid form into a wellbore by pumping the slurry down the interior of the casing and forcing the slurry to flow from the bottom of the casing back upward between the casing and the wellbore. After the cement has been pumped into the wellbore, it must be allowed to cure for a certain period of time that can vary between 12-72 hours.
By evaluating the wellbore and formation characteristics, a person skilled in the art can determine with a good deal of precision the preferred cement density to use during the cement job to most effectively protect the casing and separate producing formations. If a cement slurry density could be maintained within a tight tolerance of .+-.0.1 lbs/gallon (PPG) of the preferred density, the probability of a successful cementing operation would be much higher. Some authors have stated that proper mixing of the cement slurry is critical to successful completion of a cementing job on a well, and have proposed systems to alleviate this problem with density control in continuous-mixing operations. See e.g. Galiana, et. al., "Cement Mixing: Better Understanding and New Hardware," Oilfield Review, (April 1991); Hitt, et. al., "Process Control of Slurry Density: Impact on Field Performance of Slurry Density," presented at the Society of Petroleum Engineers' Production Operations Symposium held in Oklahoma City, Okla., Apr. 7-9, 1991; O'Neill, et. al., "New Slurry Mixer Improves Density Control in Cementing Operations," presented at the Society of Petroleum Engineers' Latin America Petroleum Engineering Conference held in Rio de Janeiro, Oct. 14-19, 1990; Wienck, et. al., "Automatic Control of Bulk Cement Tank Levels," presented at the 24th Annual Offshore Technology Conference in Houston, Tex., May 4-7 1992; and Stegemoeller, et. al., "Automatic Density Control and High Specific Mixing Energy Deliver Consistent High-Quality Cement Slurries," presented at the 24th Annual Offshore Technology Conference in Houston, Tex., May 4-7, 1992.
Unfortunately, the prior art continuous-mix cementing systems have been unable to guarantee the density of the cement slurry within an acceptable tolerance level. Most prior art cementing systems are subject to a wide fluctuation in cement slurry density. See the discussion in Galiana, et. al., "Cement Mixing: Better Understanding and New Hardware," Oilfield Review, (April 1991). Even the systems developed more recently have encountered difficulty in obtaining a slurry density within .+-.0.1 lbs/gallon. Id. One of the reasons for this variance is that the meters and valves used in the mixing and density control systems typically are designed to be within a predetermined accuracy. Consequently, a certain amount of error is common in most meters. This is especially true with respect to the dry cement delivery system. Compounding this problem is the fact that many density control systems attempt to obtain a desired density by fixing the amount of dry cement to be delivered, while adjusting the rate at which water is input based upon feedback from a density sensor. See e.g. Stegemoeller, et. al. "Automatic Density Control and High Specific Mixing Energy Deliver Consistent High-Quality Cement Slurries," presented at the 24th Annual Offshore Technology Conference in Houston, Tex., May 4-7, 1992. The meters and valves associated with the dry cement delivery system have a relatively large error associated with them that makes any control system suspect that is based upon setting the delivery of dry cement to a fixed level. This error results from a number of factors such as the tendency of the dry cement to coagulate. Because of the difficulty in handling and supplying dry cement during a cementing operation, it is very difficult to maintain a constant slurry density.
Because of the inherent inaccuracy in all of the meters and valves typically used in an automatic density system, and especially those related to the dry cement delivery system, it is extremely difficult to design a system that can very accurately and precisely control the density of a cement slurry. It is an object of the present invention to automatically control the density of the cement slurry obtained in a continuous-mixing application to within .+-.0.1 lbs/gallon (PPG) of the desired density for cementing operations that is relatively independent of the error inherent in the meters and valves used in the system and especially those related to the dry cement delivery system.